System for collecting carbon dioxide in flue gas

ABSTRACT

A system for collecting carbon dioxide in flue gas includes a stack that discharges flue gas discharged from an industrial facility to outside, a blower that is installed at the downstream side of the stack and draws the flue gas therein, a carbon-dioxide collecting device that collects carbon dioxide in the flue gas drawn in by the blower, and a gas flow sensor arranged near an exit side within the stack. A drawing amount of the flue gas by the blower to the carbon-dioxide collecting device is increased until an flow rate of the flue gas from the stack becomes zero in the gas flow sensor, and when the discharged amount of flue gas from the stack becomes zero, drawing in any more than that amount is stopped, and the carbon dioxide in the flue gas is collected while the flue gas is drawn in by a substantially constant amount.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 12/368,474 filed on Feb. 10, 2009, which is now U.S. Pat. No. 8,623,286 B2, the entire contents of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system for collecting carbon dioxide in flue gas, capable of stably processing all the carbon dioxide in the flue gas discharged from an industrial facility such as a gas turbine, a furnace, or a boiler.

2. Description of the Related Art

Conventionally, to collect carbon dioxide in flue gas, for example, a system is adopted when an amine-based absorbing solvent is used as a CO₂ absorbing solvent to remove and collect CO₂ from flue gas; firstly, a process is performed to bring the CO₂ absorbing solvent into contact with the flue gas in an absorption column, and thereafter the CO₂ absorbing solvent that has absorbed CO₂ is heated in a regeneration column; secondly, CO₂ is freed and the CO₂ absorbing solvent is regenerated, and circulated in the absorption column again, and finally, reused (for example, see Japanese Patent Application Laid-open No. H3-193116).

FIG. 9 is an example of a conventional system for collecting Co₂ in flue gas. As shown in FIG. 9, a conventional CO₂ collecting system 1000 includes an flue gas cooling device 1004 that cools flue gas 1002 containing CO₂ discharged from an industrial facility 1001 such as a boiler and a gas turbine by cooling water 1003, a CO₂ absorption column 1006 that brings the flue gas 1002 containing cooled CO₂ into contact with a CO₂ absorbing solvent 1005 absorbing CO₂ to remove the CO₂ from the flue gas 1002, and a regeneration column 1008 that releases the CO₂ from a CO₂ absorbing solvent (rich solvent) 1007 absorbing the CO₂ to regenerate the CO₂ absorbing solvent. In this device, a regenerated CO₂ absorbing solvent (lean solvent) 1009 from which the CO₂ is removed in the regeneration column 1008 is recycled as the CO₂ absorbing solvent in the CO₂ absorption column 1006. The CO₂ absorption column 1006 and the regeneration column 1008 configure a CO₂ collecting device 1030.

In a CO₂ collection method using the conventional CO₂ collecting device 1030, the flue gas 1002 containing CO₂, discharged from the industrial facility such as a boiler and a gas turbine, is firstly sent to the flue gas cooling device 1004 after pressure is raised by an flue gas fan 1010, cooled by cooling water 1003, and thereafter sent to the CO₂ absorption column 1006.

In the CO₂ absorption column 1006, the flue gas 1002 is countercurrently contacted with, for example, the CO₂ absorbing solvent 1005 based on an amine-based absorbent, and the CO₂ in the flue gas 1002 is absorbed in the CO₂ absorbing solvent 1005 due to a chemical reaction (R—NH₂+H₂O+CO₂→R—NH₃HCO₃), and flue gas 1011 from which the CO₂ is removed is released from the system. The CO₂ absorbing solvent 1007 absorbing the CO₂ is also called a rich solvent. Pressure is raised for the rich solvent 1007 by a rich solvent pump 1012, and heated by a rich/lean solvent heat exchanger 1013 by the CO₂ absorbing solvent (lean solvent) 1009 regenerated by removing CO₂ in the regeneration column 1008 described later, and supplied to the regeneration column 1008.

The rich solvent 1007 released from the upper part of the regeneration column 1008 to the interior of the regeneration column 1008 receives heat from water vapor generated inside the regeneration column 1008 and a large amount of CO₂ is discharged. The CO₂ absorbing solvent from which a portion or a large amount of CO₂ is released inside the regeneration column 1008 is called a semi-lean solvent. The semi-lean solvent becomes a CO₂ absorbing solvent from which almost all of the CO₂ is removed by the time it reaches the lower portion of the regeneration column 1008. The absorbing solvent regenerated by removing almost all of the CO₂ is called a lean solvent. A re-boiler 1014 heats the lean solvent by using steam. Meanwhile, CO₂ gas 1015 together with water vapor released from the rich solvent and the semi-lean solvent inside the regeneration column 1008 is derived from the head top part of the regeneration column 1008, the water vapor is condensed by an overhead condenser 1016, water is separated by a separation drum 1017, and CO₂ gas 1018 is released from the system and collected. The water separated by the separation drum 1017 is supplied by a condensed-water circulating pump 1019 to the upper part of regeneration column 1008. The regenerated CO₂ absorbing solvent (lean solvent) 1009 is cooled by the rich solvent 1007 by the rich/lean solvent heat exchanger 1013, which is followed by raising the pressure by a lean solvent pump 1020, and the resultant water is cooled by a lean solvent cooler 1021, and thereafter supplied to the CO₂ absorption column 1006.

In FIG. 9, reference numeral 1001 a is a stack flue of the industrial facility 1001 such as a boiler and a gas turbine, and 1001 b is a stack having a damper on the inside. There are cases that the CO₂ collecting device is installed after the system is completed to collect CO₂ from an existing source of the flue gas and cases that it is simultaneously attached to a new source of the flue gas.

As the conventional effective utilization of the carbon dioxide in flue gas, some of the carbon dioxide in flue gas is merely collected to produce carbon dioxide for carbonated drinks and dry ice. However, the greenhouse effect caused by carbon dioxide is recently pointed out as one of the causes of global warming. Measures are becoming an urgent necessity internationally to protect the global environment, and also, the source of generating the carbon dioxide affects every field of human activity, which burns fossil fuels, and demands of restricting the discharge tend are becoming even stronger. Along with this tendency, in power generation facilities such as thermal power stations that use a large amount of fossil fuels, there have been a method in which flue gas of industrial facilities such as a boiler and a gas turbine is brought into contact with a CO₂ absorbing solvent, all of CO₂ in the flue gas is removed and collected, and a method for storing the collected CO₂ without releasing it into the atmosphere.

As described above, in the conventional CO₂ collecting system 1000, there has been proposed that, when collecting all of carbon dioxide in flue gas, a containing unit such as a valve or damper that can be opened and closed inside the stack 1001 b, as shown in FIG. 9, is installed to close and stop during the operation of the CO₂ collecting device and to release when the operation of the CO₂ collecting device is stopped while the source of the flue gas is kept operating.

However, at the time of stopping the operation of the CO₂ collecting device, unless an operation such as containment of the valve, damper or the like, which closes the interior of the stack or releasing is surely performed, discharge of flue gas is not performed smoothly, and there are occasions that the industrial facilities (such as gas turbines) at the upstream side are adversely affected.

Further, in a turbine facility that generates electricity of 200,000 kilowatts in which 3000 ton of carbon dioxide are processed in each day, an amount of discharged flue gas becomes enormous and a stack having, for example, a diameter of 7 to 10 meters is required. Under such circumstances, a facility of a containing unit such as a valve and a damper that contains the flue gas needs to be larger.

Accordingly, a system capable of drawing in substantially all of a large amount of flue gas to a carbon-dioxide collecting device in a simple, stable, and safe manner has been desired.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partially solve the problems in the conventional technology.

According to an aspect of the present invention, a system for collecting carbon dioxide in flue gas includes: a stack that discharges flue gas discharged from an industrial facility to outside; a blower that is installed at a downstream side of the stack and draws the flue gas therein; a carbon-dioxide collecting device that collects carbon dioxide in the flue gas drawn in by the blower; and a gas flow sensor that is arranged near an exit side within the stack and measures a gas flow rate. A drawing amount of the flue gas to the carbon-dioxide collecting device by the blower is increased until an flow rate of the flue gas from the stack becomes zero in the gas flow sensor, and when the flow rate of the flue gas from the stack becomes zero, drawing in any more than the amount is stopped, and the carbon dioxide in the flue gas is collected while drawing in the flue gas by a substantially constant amount.

According to another aspect of the present invention, a system for collecting carbon dioxide in flue gas includes: a stack that discharges flue gas discharged from an industrial facility to outside; a blower that is installed at a downstream side of the stack and draws the flue gas therein; a carbon-dioxide collecting device that collects carbon dioxide in the flue gas drawn in by the blower; and a sensor that is arranged at least one location in a stack flue at a downstream side of the stack within the stack and measures gas temperature or gas type. A drawing amount of the flue gas by the blower to the carbon-dioxide collecting device is increased until the gas temperature or a concentration of the gas type changes in the sensor, and when the gas temperature or the concentration of the gas type changes, drawing in any more than the amount is stopped, and the carbon dioxide in the flue gas is collected while drawing in the flue gas by a substantially constant amount.

According to still another aspect of the present invention, a system for collecting carbon dioxide in flue gas includes: a stack that discharges flue gas discharged from an industrial facility to outside; a blower that is installed at a downstream side of the stack and draws the flue gas therein; a carbon-dioxide collecting device that collects carbon dioxide in the flue gas drawn in by the blower; and a plurality of sensors that are arranged in an stack flue at an upstream side of the stack and at a downstream side of the stack and measure gas temperature or a concentration of gas type. A drawing amount of the flue gas by the blower to the carbon-dioxide collecting device is increased until a difference in the gas temperature or the concentration of the gas type is generated, and when the difference is generated, drawing in any more than the amount is stopped, and the carbon dioxide in the flue gas is collected while drawing in the flue gas by a substantially constant amount.

In the system for collecting carbon dioxide in flue gas, the gas type may be oxygen or carbon dioxide.

The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system for collecting carbon dioxide in flue gas according to a first embodiment of the present invention;

FIG. 2 depicts a relationship between a discharged amount of flue gas from a stack and a drawing amount of flue gas in the first embodiment;

FIG. 3A is a schematic diagram of a system for collecting carbon dioxide in flue gas according to a second embodiment of the present invention;

FIG. 3B is a schematic diagram of another system for collecting carbon dioxide in flue gas according to the second embodiment;

FIG. 3C is a schematic diagram of still another system for collecting carbon dioxide in flue gas according to the second embodiment;

FIG. 4 depicts a relationship between discharged amount of flue gas from a stack and a drawing amount of flue gas in the second embodiment;

FIG. 5A is a schematic diagram of a system for collecting carbon dioxide in flue gas according to a third embodiment of the present invention;

FIG. 5B is a schematic diagram of another system for collecting carbon dioxide in flue gas according to the third embodiment;

FIG. 6 depicts a relationship between oxygen concentration from a stack and a drawing amount of flue gas in the third embodiment;

FIG. 7A is a schematic diagram of a system for collecting carbon dioxide in flue gas according to a fourth embodiment of the present invention;

FIG. 7B is a schematic diagram of another system for collecting carbon dioxide in flue gas according to the fourth embodiment;

FIG. 8 depicts a relationship between oxygen concentration of flue gas from a stack and a drawing amount of flue gas in the fourth embodiment; and

FIG. 9 is a schematic diagram of a conventional system for collecting carbon dioxide in flue gas.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is explained below in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments. In addition, constituent elements in the following embodiments include elements that readily occur to those skilled in the art or substantially equivalent elements.

A system for collecting carbon dioxide in flue gas according to a first embodiment of the present invention is explained with reference to the drawings.

FIG. 1 is a schematic diagram of the system for collecting carbon dioxide in flue gas according to the first embodiment.

As shown in FIG. 1, a system for collecting carbon dioxide in flue gas 10-1 according to the first embodiment includes a stack 13 that discharges flue gas 12 discharged from an industrial facility 11 to outside, a blower 14 that is installed at the downstream side of the stack 13 and draws the flue gas 12 therein, a carbon-dioxide collecting device 15 that collects carbon dioxide in the flue gas 12 drawn in by the blower 14, and a gas flow sensor S1 arranged near an exit side within the stack 13. In the gas flow sensor S1, a drawing amount of the flue gas 12 by the blower 14 to the carbon-dioxide collecting device 15 is increased until an exhaust flow rate of the flue gas from the stack 13 becomes zero, and when the discharged amount of flue gas from the stack 13 becomes zero, drawing in any more than that amount is stopped and the carbon dioxide in the flue gas is collected while the flue gas 12 is drawn in by a substantially constant amount.

The carbon-dioxide collecting device 15 is not particularly limited and can be any device that collects carbon dioxide, similar to the CO₂ collecting device shown in the FIG. 9.

In the first embodiment, the exhaust flow rate to the outside of the flue gas 12 from the stack 13 is monitored all the time in the gas flow sensor S1.

When the drawing amount of the flue gas 12 by the blower 14 to the carbon-dioxide collecting device 15 is increased until the exhaust flow rate from the stack 13 becomes zero, the discharged amount of the flue gas 12 to the outside from the stack declines slowly as shown in FIG. 2. When the discharged amount of flue gas from the stack 13 becomes zero, drawing in any more than that amount by the blower 14 is stopped, and the carbon dioxide in the flue gas 12 is collected by the carbon-dioxide collecting device 15 while drawing in the flue gas 12 by the blower 14 while maintaining the drawing amount to become approximately zero (substantially constant amount).

Accordingly, all of the flue gas from the industrial facility 11 can be stably drawn into the carbon-dioxide collecting device 15 and all of the carbon dioxide in flue gas can be collected.

Further, in the carbon-dioxide collecting device 15, since air is mixed in the flue gas which is a target to be processed, malfunctions is not generated even when outside atmosphere is drawn into the carbon-dioxide collecting device 15 from the stack 13.

In the present invention, the industrial facility is not particularly limited, and examples thereof can include a boiler, a combustion furnace, and a gas turbine facility, which generate carbon dioxide. When the gas turbine facility is used, a heat recovery steam generator (HRSG) that collects heat having high temperature (about 580° C.) of the flue gas 12 discharged from a gas turbine can be installed.

A system for collecting carbon dioxide in flue gas according to a second embodiment of the present invention is explained next with reference to the drawings.

FIG. 3A is a schematic diagram of the system for collecting carbon dioxide in flue gas according to the second embodiment.

As shown in FIG. 3A, a system for collecting carbon dioxide in flue gas 10-2A according to the second embodiment includes the stack 13 that discharges the flue gas 12 discharged from the industrial facility 11 to the outside, the blower 14 that is installed at the downstream side of the stack 13 and draws the flue gas 12 therein, the carbon-dioxide collecting device 15 that collects carbon dioxide in the flue gas 12 drawn in by the blower 14, and a gas temperature sensor S2 arranged within the stack 13. In the gas temperature sensor S2, the drawing amount of the flue gas 12 by the blower 14 to the carbon-dioxide collecting device 15 is increased until the gas temperature declines, and when the gas temperature declines, drawing in any more than that amount is stopped and the carbon dioxide in the flue gas is collected while the flue gas 12 is drawn in by a substantially constant amount.

That is, in the gas temperature sensor S2, the exhaust temperature of the flue gas to be discharged from the stack 13 to the outside is monitored all the time by the gas temperature sensor S2.

Even when the drawing amount of the flue gas 12 by the blower 14 to the carbon-dioxide collecting device 15 is increased, the exhaust temperature from the stack 13 is almost maintained at constant temperature (100 to 180° C.) up to a certain time point as shown in FIG. 4. However, when atmosphere from outside the stack 13 flows in within the stack 13, the flue gas 12 is cooled by the atmosphere, and the temperature declines. Additionally, immediately before the temperature change starts (a point indicated by an arrow in FIG. 4), drawing in the flue gas 12 by the blower 14 any more than that amount is stopped, and the carbon dioxide in the flue gas 12 is collected by the carbon-dioxide collecting device 15 while drawing in the flue gas 12 by the blower 14 while maintaining the stopped drawing amount (substantially constant amount).

Accordingly, all of the flue gas from the industrial facility 11 can be stably drawn into the carbon-dioxide collecting device 15 and all of the carbon dioxide in flue gas can be collected.

FIG. 3B depicts another system for collecting carbon dioxide in flue gas 10-2B according to the second embodiment. A gas temperature sensor S3 is arranged before the blower 14 at the downstream side of the stack, and similarly, the gas temperature is measured and the flue gas is drawn in.

By using the gas temperature sensor S3 to control the drawing amount while monitoring the gas temperature in the same way as the gas temperature sensor S2, all of the flue gas from the industrial facility 11 can be stably drawn into the carbon-dioxide collecting device 15 and all of the carbon dioxide in the flue gas can be collected.

FIG. 3C depicts still another system for collecting carbon dioxide in flue gas 10-2C according to the second embodiment. The gas temperature sensor S3 is arranged before the blower 14 and at the downstream side of the stack 13, and a gas temperature sensor S4 also is arranged within a stack flue at the upstream of the stack 13.

In the gas temperature sensors S3 and S4, the drawing amount of the flue gas 12 by the blower 14 to the carbon-dioxide collecting device 15 is increased until a difference in the flue gas temperature is generated, and when the difference in the gas temperatures is generated, drawing in any more than that amount is stopped and the carbon dioxide in flue gas is collected while drawing in the flue gas by a substantially constant amount.

Accordingly, all of the flue gas from the industrial facility 11 can be stably drawn into the carbon-dioxide collecting device 15.

A system for collecting carbon dioxide in flue gas according to a third embodiment of the present invention is explained next with reference to the drawings.

FIG. 5A is a schematic diagram of the system for collecting carbon dioxide in flue gas according to the third embodiment.

As shown in FIG. 5A, a system for collecting carbon dioxide in flue gas 10-3A according to the third embodiment includes the stack 13 that discharges the flue gas 12 discharged from the industrial facility 11 to the outside, the blower 14 that is installed at the downstream side of the stack 13 and draws the flue gas 12 therein, the carbon-dioxide collecting device 15 that collects carbon dioxide in the flue gas 12 drawn in by the blower 14, and an O₂ sensor S5 arranged within the stack 13. In the O₂ sensor S5, the drawing amount of the flue gas 12 by the blower 14 to the carbon-dioxide collecting device 15 is increased until the oxygen gas concentration is raised, and when the oxygen gas concentration is raised, drawing in any more than that amount is stopped and the carbon dioxide in the flue gas 12 is collected while the flue gas 12 is drawn in by a substantially constant amount.

That is, in the O₂ sensor S5, the O₂ sensor S5 monitors all the time the oxygen concentration of the flue gas to be discharged from the stack 13 to the outside.

Even when the drawing amount of the flue gas 12 by the blower 14 to the carbon-dioxide collecting device 15 is increased, the oxygen concentration of flue gas from the stack is maintained almost at constant (in a case of the flue gas from a boiler, it is 2 to 5%, and in a case of the flue gas from a gas turbine, it is 12 to 15%) up to a certain time point as shown in FIG. 6. However, when atmosphere from outside of the stack 13 flows into the stack, oxygen in the atmosphere is mixed in the flue gas 12, and the oxygen concentration is raised. Immediately before the concentration change in oxygen starts (a point indicated by an arrow in FIG. 6), drawing in any more flue gas 12 by the blower 14 is stopped, and carbon dioxide in the flue gas 12 is collected by the carbon-dioxide collecting device 15 while drawing in the flue gas 12 by the blower 14, maintaining the stopped drawing amount (substantially constant amount) at the same time.

Accordingly, all of the flue gas from the industrial facility 11 can be stably drawn into the carbon-dioxide collecting device 15 and all of the carbon dioxide in flue gas can be collected.

In FIG. 5A, an O₂ sensor S6 is installed at the upstream side of the blower 14 at the downstream side of the stack 13, and the oxygen concentration can be measured by the O₂ sensor S6 instead of the O₂ sensor S5. At this time, the O₂ sensor S5 can be used together so that the oxygen concentration can be measured by the both sensors S5 and S6.

FIG. 5B depicts another system for collecting carbon dioxide in flue gas 10-3B according to the third embodiment. An O₂ sensor S6 is arranged at the downstream side of the stack 13 and before the blower 14, and an O₂ sensor S7 is also arranged within the stack flue at the upstream of the stack 13.

In the O₂ sensors S6 and S7, the drawing amount of the flue gas 12 by the blower 14 to the carbon-dioxide collecting device 15 is increased until a difference in the oxygen concentration in the flue gas is generated, and when the difference in the oxygen concentration is generated, drawing in any more than that amount is stopped and carbon dioxide in the flue gas is collected while drawing in the flue gas by a substantially constant amount.

Accordingly, all of the flue gas from the industrial facility 11 can be stably drawn into the carbon-dioxide collecting device 15.

A system for collecting carbon dioxide in flue gas according to a fourth embodiment of the present invention is explained next with reference to the drawings.

FIG. 7A is a schematic diagram of the system for collecting carbon dioxide in flue gas according to the fourth embodiment.

As shown in FIG. 7A, a system for collecting carbon dioxide in flue gas 10-4A according to the fourth embodiment includes the stack 13 that discharges the flue gas 12 discharged from the industrial facility 11 to the outside, the blower 14 that is installed at the downstream side of the stack 13 and draws the flue gas 12 therein, the carbon-dioxide collecting device 15 that collects carbon dioxide in the flue gas 12 drawn in by the blower 14, and a CO₂ sensor S8 arranged within the stack 13. In the CO₂ sensor S8, the drawing amount of the flue gas 12 drawn in by the blower 14 to the carbon-dioxide collecting device 15 is increased until the carbon dioxide gas concentration is raised, and when the carbon dioxide gas concentration is raised, drawing in any more than that amount is stopped, and the carbon dioxide in the flue gas is collected while the flue gas is drawn in by a substantially constant amount.

That is, in the CO₂ sensor S8, the CO₂ sensor S8 monitors all the time the carbon dioxide concentration of the flue gas to be discharged from the stack 13 to the outside.

Even when the drawing amount of the flue gas 12 by the blower 14 to the carbon-dioxide collecting device 15 is increased, the carbon dioxide concentration of flue gas from the stack is maintained almost at constant (in a case of the flue gas from a boiler, it is 8 to 14% and in a case of the flue gas from a gas turbine, it is 3.5 to 4%) up to a certain time point as shown in FIG. 6. However, when atmosphere flows in the stack from outside of the stack 13, carbon dioxide in the atmosphere is mixed in the flue gas 12, and the carbon dioxide concentration is raised. Immediately before the concentration change of the carbon dioxide starts (a point indicated by an arrow in FIG. 8), drawing in any more flue gas 12 by the blower 14 is stopped, and the carbon dioxide in the flue gas 12 is collected by the carbon-dioxide collecting device 15 while drawing in the flue gas 12 by the blower 14 while maintaining the stopped drawing amount (substantially constant amount).

Accordingly, all of the flue gas from the industrial facility 11 can be stably drawn into the carbon-dioxide collecting device 15 and all of the carbon dioxide in flue gas can be collected.

In FIG. 7A, a CO₂ sensor S9 is installed at the upstream side of the blower 14 at the downstream side of the stack 13, and the carbon dioxide concentration can be measured by the CO₂ sensor S9 instead of the CO₂ sensor S8. At this time, the CO₂ sensor S8 can be used together so that the oxygen concentration can be measured by both the sensors S8 and S9.

FIG. 7B depicts another system for collecting carbon dioxide in flue gas 10-4B according to the fourth embodiment. The CO₂ sensor S9 is arranged at the downstream side of the stack 13 and before the blower 14, and a CO₂ sensor S10 is also arranged in the stack flue at the upstream of the stack 13.

In the CO₂ sensors S9 and S10, the drawing amount of the flue gas 12 by the blower 14 to the carbon-dioxide collecting device 15 is increased until a difference in the carbon dioxide concentration in the flue gas is generated, and when the difference in the carbon dioxide concentration is generated, drawing in any more than that amount is stopped, and the carbon dioxide in flue gas is collected while drawing in the flue gas by a substantially constant amount.

Accordingly, all of the flue gas from the industrial facility 11 can be stably drawn into the carbon-dioxide collecting device 15.

To achieve more accurate control, the first to fourth embodiments can be appropriately combined to use a plurality of different sensors and to perform the control.

According to the present invention, substantially all of a large amount of flue gas can be drawn into a carbon-dioxide collecting device with a simple facility and in a stable and safe manner.

Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth. 

What is claimed is:
 1. A system for collecting carbon dioxide in flue gas, comprising: a stack that has an entrance and an exit of the flue gas, the exit being arranged at a top of the stack, and the entrance being arranged at a lower portion of the stack and, connected to an industrial facility through a stack flue for taking in the flue gas to be discharged to outside from the exit; a carbon-dioxide collecting device that is connected to the lower portion of the stack through a flue gas passage and collects carbon dioxide in the flue gas; a blower that is installed in the flue gas passage between the stack and the carbon-dioxide collecting device to draw in the flue gas to the carbon-dioxide collecting device; a plurality of sensors configured to measure gas concentration, the gas concentration being one of oxygen concentration of the flue gas and carbon dioxide concentration of the flue gas, one of the plurality of sensors being arranged in the stack flue and another one of the plurality of sensors being arranged in the flue gas passage between the stack and the blower; and a controller connected to the blower and the plurality of sensors, the controller receiving signals corresponding to the gas concentration from the plurality of sensors, wherein the controller is configured to control the blower to increase a drawing amount of the flue gas to the carbon-dioxide collecting device until a difference between measured values of the one of the plurality of sensors and the another one of the plurality of sensors in the gas concentration is generated due to atmosphere flowing into the stack from the exit of the flue gas, and the controller is configured to stop drawing in any more than the amount immediately before the difference between the measured values of the one of the plurality of sensors and another one of the plurality of sensors in the gas concentration is generated, and to control the blower to draw in the flue gas by a substantially constant amount. 